You may have wondered why drywall screws are black and assumed the color came from paint or some kind of decorative dip. It is a reasonable guess, but the black coating on your drywall screws is not paint. It’s the byproduct of a chemical coating that protects drywall screws and makes them easier to install.
What Is the Black Phosphate Coating?
Black phosphate is a chemical conversion coating. After the screws are made, they are submerged in a heated phosphoric acid bath containing manganese or iron salts. The acid reacts with the steel surface, which grows a thin, crystalline phosphate layer that is chemically bonded to the metal underneath.
The coating won’t flake or peel off the way paint can because it chemically changes the metal surface. Drywall screws are also typically given a light coat of oil on top of the black phosphate coating. That’s why new screws have a slightly slick feel to them.
What the Coating Does for Drywall Screw Buyers
The phosphate layer has three main jobs: corrosion resistance, better adhesion, and easier installation.
Corrosion Resistance
Bare steel starts to rust in hours in a damp environment. The phosphate layer slows that down significantly. It gives the screw enough protection to sit in inventory for a few months and last for decades inside a finished wall in normal household conditions.
Better Mud Adhesion
The phosphate layer is porous at a microscopic level. It has a rough surface texture that joint compound can bind to when the screw head is mudded over. A smooth bare-metal or plated head is harder for mud to grip, which can lead to finish defects over the head.
Easier Driving
Phosphate coatings have a low coefficient of friction, especially once they are oiled. Phosphate-coated drywall screws generate less heat and less resistance as they’re driven, reducing cam-out and helping bits last longer.
Black Phosphate vs. Other Coatings
Black phosphate isn’t the only coating you’ll find on construction screws. There are many more, each of which is engineered for a specific application or environment.
- Zinc plating is shinier, and it offers more corrosion protection in damp environments. It’s common on interior wood and trim screws. But it’s not ideal for drywall because the smooth surface doesn’t bond as well with joint compound.
- Black oxide is sometimes confused with black phosphate because the color is similar. But black oxide offers less corrosion resistance and a smoother surface. It is more common on machine screws and tooling than on construction fasteners.
- Gray or zinc phosphate uses zinc instead of manganese and offers slightly better corrosion resistance at a higher cost.
Where Black Phosphate Belongs and Where It Doesn’t
Black phosphate is the best choice for interior drywall and similar dry, conditioned applications. However, it is not the right coating for areas with sustained moisture or anywhere the screw will contact pressure-treated lumber.
ACQ and other modern treatment chemistries are corrosive to plain phosphated steel and will eat through the coating. For those jobs, an exterior-grade coating like RustGuard or a stainless steel fastener is the better option.
Drywall Screws for Every Project
Sure Drive black phosphate drywall screws are the everyday choice for interior drywall work. The full line is available in coarse and fine thread, standard and Type-17 points, in sizes and pack counts that cover your customers’ everyday needs. Browse our full range of Sure Drive fasteners.
